Low-cost radio altimeter

ABSTRACT

A relatively low-cost FMCW radio altimeter includes a voltage-controlled oscillator based upon a GaAs FET. The oscillator produces, for example, a 4.3 GHz microwave signal that is modulated with a triangular wave having a pin-selectable modulation frequency. The modulated signal is amplified by a buffer amplifier, as well as a power amplifier, and connected to an RF output terminal by way of a plurality of microstrips. Reflected signals are received at an RF input and are coupled to a mixer by way of a low-noise amplifier. The modulation signal is also applied to the mixer by way of a microstrip coupling device to produce an audio output signal whose frequency is proportional to the altitude above ground. The gains of all of the amplifiers are selected to eliminate the need for hand-tuning of the microstrips and to enable the use of a glass/epoxy circuit board. The output of the radio altimeter is an audio signal whose frequency is proportional to the height above ground, which enables direct digital conversion by way of a frequency-to-binary converter.

CROSS REFERENCE TO RELATED APPLICATIONS

This case is related to a copending application Ser. No. 08/599,735,filed on even date, entitled "Terrain Warning System" by John Poe, JohnSnyder Jr., Rory Kestner, Paul Williams, Barry McAnulty and BrianFriedrich, Docket No. 543-95-007 (5806/59142).

BACKGROUND OF THE INVENTION

The present invention relates to a relatively low-cost radio altimeterand more particularly to a FMCW radio altimeter.

Many aircraft operated throughout the world are generally not equippedwith a radio altimeter; a device that indicates the height of theaircraft above ground, because of the cost involved. Such radioaltimeters are known to enhance the safety of the flight by providingthe pilot with vital information relative to the aircraft's positionrelative to ground. Such aircraft are normally provided with abarometric altimeter, which measures the barometric air pressure andprovides a signal relative to the height above sea level. Since thebarometric altimeters provide no information relative to the heightabove ground, pilots must constantly be aware of their geographiclocation and also the topology of the terrain along the flight path toavoid impact with terrain. Many aircraft accidents have occurred inwhich the barometric altitude of the aircraft was maintained constant inwhich the pilot was unaware that the terrain was rising along the flightpath. A radio altimeter solves such a problem by providing the pilot ofan aircraft with altitude data based upon the height of the aircraftabove the terrain. Unfortunately, several barriers exist to equippingthe world's aircraft with radio altimeters, including the cost ofequipment, the physical size and weight of the equipment, as well as thecost of installing the equipment on an aircraft.

Radio altimeter research dates back to the 1940s. Both pulse-type radioaltimeters and frequency-modulated continuous-wave (FMCW) are known.Pulse-type radio altimeters typically measure the actual time requiredfor a transmitted pulse to travel to the ground and return to areceiving antenna on the aircraft. Although such pulse-type radioaltimeters provide excellent performance, these radio altimeters arevery expensive and often quite large and heavy. As such, FMCW radioaltimeters have been developed. Early FMCW radio altimeters used klystontubes to generate microwave energy. Such klyston tube-based altimeterswere relatively expensive, required expensive high-voltage powersupplies and required very careful and skillful assembly and alignment.Unfortunately, performance of such klyston tube-based radio altimeterswas marginal, while the reliability was relatively poor. However,klystons remained the only practical method to generate microwave energyuntil the 1970s. In the 1970s semiconductor technology had advanced to apoint where microwave energy could be generated using special, extremelyhigh-frequency bipolar transistors. Additionally, the semiconductortechnology also produced improved, high-frequency diodes, which allowedthe construction of solid-state FMCW radio altimeters. Unfortunately,such solid-state radio altimeters have required the use of relativelyexpensive, Teflon-based printed circuit board material and polished goldplating on metal surfaces to minimize noise. In addition, the microwavetransistors and diodes must be hand tuned, requiring countless hours ofskillful and technical alignment. An example of such a radio altimeteris an AlliedSignal KRA10-10A radio altimeter, which includes tuningstubs for proper alignment. Skilled technicians using sharp, smallknives are required to carefully scrape the stubs to tune the circuits.Such a process is relatively time consuming and, once tuned, the boardis optimized only for the exact microwave transistors and diodesinstalled. Should the microwave transistor or diodes fail, the entiremicrostrip transmitter/receiver unit is replaced; and the tuning processis redone. In addition, such Teflon circuit boards are relativelyfragile and are relatively easy to damage.

SUMMARY OF THE INVENTION

The present invention solves the problems in the prior art and providesa relatively low-cost FMCW radio altimeter.

According to one aspect of the present invention, the radio altimeterdoes not require hand-tuning of tuning stubs in the microwave circuit.

According to another aspect of the present invention, to the radioaltimeter circuit can be fabricated on a glass/epoxy printed circuitboard.

Briefly, the present invention relates to a FMCW radio altimeter thatincludes a voltage-controlled oscillator based upon a GaAs FET. Theoscillator produces, for example, a 4.3 GHz microwave signal that ismodulated with a triangular wave having a pin-selectable modulationfrequency. The modulated signal is amplified by a buffer amplifier, aswell as a power amplifier, and connected to an RF output terminal by wayof a plurality of microstrips. Reflected signals are received at an RFinput and are coupled to a mixer by way of a low-noise amplifier. Themodulation signal is also applied to the mixer by way of a microstripcoupling device to produce an audio output signal whose frequency isproportional to the altitude above ground. The gains of all of theamplifiers are selected to eliminate the need for hand-tuning of themicrostrips and to enable the use of a glass/epoxy circuit board. Theoutput of the radio altimeter is an audio signal whose frequency isproportional to the height above ground, which enables direct digitalconversion by way of a frequency-to-binary converter.

The radio altimeter of the present invention thus provides the safetybenefits of radio altimeter height above terrain information but at asize, cost and weight appropriate for the operation of a greater numberof users.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a top level, functional block diagram of a flight safetysystem according to an embodiment of the present invention;

FIG. 1B is a block diagram of a flight safety system constructedaccording to one embodiment of the present invention;

FIG. 2 is a block diagram of a digital input for aircraft configurationinformation according to an embodiment of the present invention;

FIG. 3 is a block diagram of an audio circuit according to an embodimentof the present invention;

FIG. 4 is a front view of a display according to an embodiment of thepresent invention;

FIG. 5A is a front view of a display circuit card according to anembodiment of the present invention;

FIG. 5B is a side view of the circuit card of FIG. 5A mounted to abezel;

FIG. 5C is a front view of the circuit card of FIG. 5A mounted in abezel;

FIG. 5D is a front view of the circuit card of FIG. 5A mounted in acircular assembly according to an alternate embodiment of the presentinvention;

FIG. 5E is a side view of the device of FIG. 5D;

FIGS. 6A-6D are front, top, side and end views of a housing suitable forcontaining and mounting the present invention;

FIGS. 7A-7C are side, plan and end views of a separate RF moduleaccording to an embodiment of the present invention;

FIG. 8 is a block diagram of a radio altimeter receiver according to oneembodiment of the present invention;

FIG. 9 is a block diagram of a radio altimeter transmitter according toan embodiment of the present invention;

FIG. 10A is a small scale view showing the whole formed by partialviews, FIGS. 10B through 10F, and indicating the positions of the partsshown to form an illustrative schematic of microstrip technologyutilized in an embodiment of the present invention;

FIG. 10B is a partial view illustrative of a schematic of microstriptechnology utilized in an embodiment of the present invention;

FIG. 10C is a partial view illustrative of a schematic of microstriptechnology utilized in an embodiment of the present invention;

FIG. 10D is a partial view illustrative of a schematic of microstriptechnology utilized in an embodiment of the present invention;

FIG. 10E is a partial view illustrative of a schematic of microstriptechnology utilized in an embodiment of the present invention;

FIG. 10F is a partial view illustrative of a schematic of microstriptechnology utilized in an embodiment of the present invention;

FIGS. 11 is a flow chart for the flight safety device according to oneembodiment of the present invention;

FIG. 12 is a flow chart for the flight safety device according to oneembodiment of the present invention;

FIG. 13 is a flow chart for the flight safety device according to oneembodiment of the present invention;

FIG. 14 is a software object diagram for the software control of theflight safety device of the present invention;

FIG. 15 is a graphical representation of the warning envelopes for the"CAUTION RISING TERRAIN", and "TERRAIN TERRAIN" warnings according to anembodiment of the present invention; and

FIG. 16 is a flow chart of the flight history process according to anembodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The radio altimeter of the present invention may be used in a flightsafety system of the present invention intended for use in generalaviation aircraft operated under Part 91 of the FAA rules andregulations. Although the system is not a ground proximity warningdevice as defined in TSO C92b, the system can provide alerts of variouspredetermined flight conditions including warnings and altitude callouts similar to ground proximity warning systems. These predeterminedflight conditions may include, for example, unacceptable proximity toterrain, unacceptable closure rate to terrain, improper aircraftconfiguration, illogical aircraft configuration/airspeed combinations,and altitude alerts. Exemplary audio messages and altitude call outs maybe, for example, "CAUTION RISING TERRAIN"; "TERRAIN, TERRAIN"; "TOO LOWFLAPS"; "TWS OK"; "MINIMUMS" and various altitude call outs, e.g.,"800"; "500"; "200"; "100" and "50". Accordingly, the flight safetydevice of the present invention may be optionally referred to as a"terrain warning system" or "TWS". This nomenclature, however, is notintended to define or otherwise limit the type of alerts that may beprovided according to the teachings of the present invention.

FIG. 1A contains a top level block diagram of a flight safety device 2constructed according to the present invention. Flight safety device 2may be powered from the aircraft electrical system 10 and processesinputs 15 and 20 representative of the aircraft's present configurationas well as an input 25 representing the aircraft height above terrain.In the embodiment of FIG. 1A, height above terrain data 25 is suppliedby a novel radio altimeter 30 described further below. Device 2processes inputs 15, 20 and 25 to provide the alerts and warnings ofvarious flight conditions. Alerts may include audible alerts supplied toexisting aircraft headphone and speaker systems 35 and 40 or variousvisual displays. Visual displays may include, for example, decisionheight and radio altitude.

FIG. 1B contains a block diagram of a flight safety system 2 accordingto one embodiment of the invention from which the interrelationship ofsystem components may be described. More detailed descriptions of systemcomponents are provided below as appropriate.

In the block diagram of FIG. 1B, system 2 operates under the control ofan 8 bit embedded controller 736. Embedded controller 736 is a generalpurpose embedded microprocessor used to process the input signals,generate the various alerts and drive a display 737. According to oneembodiment of the present invention, embedded controller 736 is an IntelModel No. 80C188EB. Alternatively, a Intel Model No. KU80C188EC20processor may be used.

Controller 736 operates in conjunction with several memory devices togenerate alerts using aircraft status and warning envelopes stored in asystem and voice memory 742. Although other memory types may be used, ina preferred embodiment of the invention, system code/voice memory 742 isa 2 Meg boot-block flash ROM device Intel Model TE28F200BX-T80. Systemcode necessary to operate the device, perform the alert and warningfunctions and boot-strap code are stored in the memory 742. Thedigitized voices for audible alerts are also stored in device 742.Provision for updating the code or voices is provided via a RS232interface. During this process, processor 736 runs out of code stored inthe protected boot-block of an additional memory.

A system random access memory 743 is used to store temporary softwarevariables during device operation. Examples of temporary variablesinclude variables that indicate the current position of the flaps orradio altitude value. RAM 743 may be a 1 Meg SRAM.

Also shown in FIG. 1B is a data memory/flight history device 745 in theform of a flash read only memory (ROM). ROM 745 is of the boot-blocktype and stores code to enable running processor 736 from this blockduring reprogramming of the system/voice memory in the field using theRS232 connection. When the device of the present invention is installedin an aircraft for the first time, the system must know the specifics ofthe aircraft configuration. For example, the system must know if theaircraft has flaps, retractable landing gear, the range of flapoperation, and other items such as optional features or altitudecall-outs. This configuration data is also stored in ROM 745. Controller736 also periodically writes flight history information to ROM 745. Thestructure and operation of the flight history/data memory are describedwith greater specificity below.

Controller 736 receives as a first input, altitude data indicative ofthe aircraft height above the terrain. In the embodiment of FIG. 1B,this altitude data is supplied from a radio altimeter shown generally bydashed lines 747. A transmit and a receive antennae 754 and 755 operateto emit and sense radio signals respectively. Optionally, a singleantenna may be used. The radio signals are emitted and processed undercontrol of a microwave transmitter/receiver and control circuit 758 toproduce the analog altitude signal which is further filtered andprocessed by signal processor circuitry 760. The design and operation ofthe radio altimeter are discussed in greater detail below.

The conditioned and filtered analog altitude signal is then converted tobinary form by the frequency to binary converter 764 of FIG. 1B.Frequency to binary converter 764 acts as a frequency counter andconverts the frequency output of processing block 760 to 14 bit binarydata that can be read by the 8 bit embedded controller 736.

Embedded controller 736 also receives as an additional input, 8 bitaircraft configuration information via sensor and modulation block 768.FIG. 2 contains a block diagram of the 8 bit configuration input ingreater detail.

Aircraft using the present invention will need to provide electricalsignals that clearly differentiate between flap positions from 0 degrees(flaps up) to less than 4 approach flaps (typically 10 degrees) andapproach flaps through fully down (typically 10 degrees and greater).These signals can come from discrete devices that are activated bymovement of the flaps such as microswitches, Opto-Transistor Switch orfrom flap position indicators.

The sensor outputs a signal 800 that can be any value from, for example,0 vdc to 28 vdc, and will vary depending on the position of the flaps(up down, or in landing configuration). This signal is scaled to a D.C.voltage by buffering and scaling circuit 804. Circuit 804 scales thevoltage to a value less than the upper input limit of 12 bit analog todigital converter 808 and buffered by means of two operationalamplifiers (not shown in FIG. 2). The first operational amplifier scalesthe input signal voltage down by a factor of 0.143. The secondoperational amplifier inverts and buffers the scaled voltage.

Circuit 804 supplies the conditioned signal 12 bit analog to digitalconverter 808 which converts the DC signal to a digital word. Thedigital word representing the flap position is input to the systemmicroprocessor 736.

The present invention can also include a landing gear sensor foraircraft equipped with retractable gear. The aircraft landing gearsignal information is processed in the same manner as the aircraft flapposition described above. A gear sensor outputs a DC voltage signal 810indicative of the gear position. A buffering and scaling circuit 812scales the voltage and buffers the signal in a manner similar to circuit804. The buffered signal is then converted to a digital word by analogto digital converter 808 for input to processor 736.

Microprocessor 736 is also coupled to the display 737 and to an audiooutput 828. Microprocessor 736 uses the configuration and altitudeinformation to generate alerts according to warning modes and proceduresto be outlined in detail below. In the preferred embodiment of theinvention, at least a portion of these alerts are audible alertsprovided through audio circuits 828.

The digitized audio voice messages, alerts, and warnings are stored insystem voice memory 799. Under control of processor 736, these messagesare sent when required, over the system data lines in an 8 bit format.FIG. 3 depicts the audio circuit in block diagram form. The 8 bitdigital message data are fed to an 8 bit digital to analog converter 850which is implemented by an "R-2R" resistive network in conjunction witha summing stage amplifier. The summing stage amplifier is shown above asanalog audio summing stage 855. Other digital/analog converters known tothose skilled in the art may also be used.

After conversion from digital to analog, the voice message data is sentto a 2 stage 4 pole audio filter 860. In one embodiment of theinvention, filter 860 has a cutoff frequency of approximately 7 Khz tokeep unwanted spurious electronic noise out of the aircraft audiosystem.

The filtered audio voice signal is then fed to two identical butseparate audio amplifiers 865, 866. Each amplifier 865, 866 has a 600ohm output impedance for matching to standard aircraft audio systems.Amplifier 865, 866 also include variable gain provided by means of adigitally controlled potentiometer installed in the feedback loop of theamplifier. The variable gain of the amplifier enables adjustment of thesignal out volume as input to the aircraft audio system.

Inputs to the two amplifier gain stages may also be grounded to disablethe audio amplifiers any time that an audio message is not being issued.This feature further assures that spurious electronic signals of afrequency in the audio range will not be introduced into the aircraftaudio system. When a voice message is being produced, the input stagesto audio amplifiers 865,866 are released from ground, thus enabling theaudio system. An audio signal mute circuit 870 may be activated by thepilot using a selector switch found on the system display.

Microprocessor 736 of FIG. 1B is used to drive display 737 and toprocess user selectable commands input through display 737. FIG. 4 is afront view of a display drawn to actual size according to one embodimentof the present invention. The audio mute switch described above may bereadily seen in FIG. 4 as reference numeral 900. Display 737 of FIG. 4also includes a mechanism 910 for setting altitude alerts or call-outs.One use of altitude call-outs is to announce decision height when flyingunder instrument flight rules. In the drawing as shown, the altitudecall-out mechanism 910 comprises a set of up and down buttons that varythe decision height as displayed in window 920. Display 737 furtherincludes a four character, seven segment display 930 used for displayingradio altitude or height above terrain to the pilot. Additionalfunctional features of display 737 are as tabulated in Table 1.

                  TABLE 1                                                         ______________________________________                                        DISPLAY FUNCTIONS AND ATTRIBUTES                                              FUNCTION                                                                              FUNCTIONAL DESCRIPTION                                                                          DISPLAY ATTRIBUTE                                   ______________________________________                                        Radio   Provides radio altitude height                                                                  4 character, 7 segment                              Altitude                                                                              data to the pilot. The least                                                                    Color -- Green or                                           significant digit of the                                                                        Orange                                                      display will always show as a                                                                   Ref. 930 FIG. 4                                             "O".              Filter overlay shall                                                          have first surface                                                            layer be diffused.                                  Setting Pressing Up or Down switches                                                                    Push button switches                                Decision                                                                              will change decision height by                                                                  1 Up & 1 Down                                       Height  50 foot intervals Ref. 910 FIG. 4                                     Decision                                                                              Displays set decision height                                                                    4 character, 5 × 7 dot                        Height  altitude          matrix Ref. 920 FIG. 4                                                        Color -- Yellow                                     Decision                                                                              Alerts pilot when decision                                                                      Yellow LED                                          Height  height set altitude has been                                                                    Ref. 940 FIG. 4                                     Light   reached                                                               INOP Light                                                                            System Status -- Indicates                                                                      Hidden Legend                                               system problem    Yellow background                                                             Black Legend                                                                  Ref. 950 FIG. 4                                     Mute Light                                                                            Audio Status -- Indicates audio                                                                 Green LED                                                   messages have been muted.                                                                       Ref. 960 FIG. 4                                     TWS Test                                                                              Pressing switch initiates a                                                                     Push Button Switch                                          Self Test of system                                                                             (Momentary SPST)                                                              Ref. 970 FIG. 4                                     Audio   Pressing switch will inhibit                                                                    Push Button Switch                                  Inhibit all audio messages                                                                              (Momentary SPST)                                                              Ref. 900 FIG. 4                                     Auto Dim                                                                              The brightness of both the                                                                      Photo Sensor                                                Radio Altitude and decision                                                                     Ref. 980 FIG. 4                                             height displays will be                                                       automatically controlled by a                                                 photo sensing element located                                                 on the face of the display. At                                                night, the Radio Altitude and                                                 decision height displays dim                                                  down to 3-4 Foot Lamberts.                                            ______________________________________                                    

Functional legends and characters will be "Backlighted" for nightvision.

In one embodiment of the invention, the display is sized to a 1/2 ATIform factor. FIGS. 5A-5E provide additional details of the displayconstruction. FIG. 5A shows a circuit card assembly 982 for containingthe functional components of the display. FIGS. 5B and 5C is a side andfront view respectively which illustrate how the display circuit card982 may be mounted in a bezel 990 which contains mounting hardware forsecuring the present invention within the instrument panel of theaircraft. A 24 pin connector 996, visible in FIG. 5B, connects thecircuit card to processor 736 and other device circuitry. FIGS. 5D and5E show an alternate embodiment of the present invention diagramming howthe present invention may be mounted in a standard 3.12 inch diameterinstrumentation well. FIG. 5E presents a side view of the mountinghardware and adapter plate 998 for securing the invention to theinstrument panel 999.

Display 737 forms the front portion of the device viewed through theinstrument panel by the pilot. FIGS. 6A-6D illustrate front, top, sideand rear views of the device housing 1001 respectively. As is evidentfrom the dimensions shown on the drawing, the novel electronic andpackaging design of the present invention enable the device to fiteasily within the confined spaces behind the instrument panel of smallergeneral aviation aircraft.

Visible in the rear view of housing 1001 is a 37 pin D-Subminiatureconnector 1005. Connector 1005 provides the electrical interface to theaircraft for the inputs and outputs listed in Table 2.

                  TABLE 2                                                         ______________________________________                                        AIRCRAFT INTERFACE CONNECTOR INPUTS AND OUTPUTS                               ______________________________________                                        Input:   Aircraft power, +28 VDC or +14 VDC                                            Aircraft ground                                                               Flap Position (Approach)                                                      Gear Down (if applicable)                                                     Modulation Frequency select --                                                This pin left unconnected sets the radio altimeter                            modulation frequency of 100 Hz. Grounding this pin                            selects the alternate frequency of 102 Hz. This                               pin should be left unconnected unless the aircraft                            is using another radio altimeter and a conflict                               between altimeters has been determined.                                       RS 232 -- provides EIA standard RS 232                                        communications to/from the device via this                                    connector. These RS 232 connections are normally                              used for aircraft installation for auto-                                      configuration, as well as for automatic test                                  procedures (ATPs).                                                   Output:  The device provides two available standard general                            aviation audio outputs each capable of delivering                             50 mW into 600 ohms.                                                 ______________________________________                                    

Connector 1005 is also represented as functional block 1005 of FIG. 1B.

In one embodiment of the present invention radio altimeter circuitry 747is collocated with the remaining circuitry of FIG. 1B inside housing1001. In an alternate embodiment of the present invention, the radioaltimeter is housed in a separate module 1051 as shown in FIGS. 7A-C.FIG. 7A shows a side view of the radio frequency module, FIG. 7B shows aplan view of the module and FIG. 7C shows the end view. Module 1051contains connectors 1052 and 1053 for coupling to antennae 1024 and1025, and a pin connector 1054 for connecting with the remainder of theflight safety device. Module 1051 is typically mounted on the belly ofthe airplane. Additional details on the construction and operation ofthe radio altimeter as well as various other system components are givenbelow.

RADIO ALTIMETER

The radio altimeter of the present invention includes both receiver andtransmitter components. FIG. 8 is a block diagram of the receiverportion of the radio altimeter constructed according to a preferredembodiment of the present invention.

Seen in FIG. 8 is a first low noise amplifier 2000. First low noiseamplifier 2000 uses micro strip technology and a single GaAs FET. Thisamplifier along with all the circuits of the low cost radio altimeterare fabricated on epoxy glass laminate. This material is low cost, andrugged, but has the disadvantages of greater variation in dielectricconstant and higher losses than Teflon based laminates. This designcompensates for these inherent disadvantages by using GaAs FETs withexcellent noise figures, and by using additional stages such that gainand loss variations are compensated for by the added stages. First lownoise amplifier 2000 uses resistive elements to load the micro stripcircuits, thus lowering their "Q". Lowering the "Q" of the micro stripcircuits increases the bandwidth of the circuits therefore making themmuch more tolerant of laminate variations and FET parameter variations.This design approach in turn allows the design of microwave circuitswhich do not require individual tuning of each stage. The broaderbandwidth, higher loss laminate, wider variation in dielectric constant,and lower "Q" will require additional stages to compensate for thereduced gain of each stage. The very small cost of the additional GaAsmicrowave FETs is more than compensated for by the cost saved on Teflonlaminate and labor to adjust and tune the microwave circuits. Thisamplifier stage includes micro strip bandpass filters to eliminateunwanted out of band signals. Further advantages of this design approachare discussed in the description of the radio altimeter transmitter.

The output of amplifier 2000 is input to a second low noise amplifier2010, similar in construction to first low noise amplifier 2000. Thisstage uses a single GaAs FET as its active element. As with first lownoise amplifier 2000, this stage uses micro strip circuits, but does notinclude micro strip bandpass filters. Resistive loading is used tocontrol the "Q" of this amplifier stage.

A third low noise amplifier 2020 receives the output of second amplifier2010. Third low noise amplifier 2020 is quite similar to second lownoise amplifier 2010. Stage 2020 also uses a single GaAs FET as itsactive element. As mentioned in connection with the design of reference2000, the inclusion of these required additional stages provide anadditional benefit. Each active stage contributes to improving isolationfrom signals such as the local oscillator signal. This isolationimprovement is important to the performance of the altimeter. Leakagesignals from the local oscillator propagate out the receiver outputwhere they are reflected by any mismatch at the antenna or transmissionline. These reflected signals return through the receiver. Because ofthe physical distance they traveled going and coming from thetransmission line, these signals appear to the balanced mixer at afrequency slightly different then the present frequency of the localoscillator. This signal would therefore produce a difference signalwhich would actually be a measure of the transmission line and itsassociated propagation delay; instead of an actual measurement of heightabove terrain. Prior radio altimeter designs often made use ofIso-Circulators in an attempt to minimize this leakage. Iso-Circulatorsadd cost to the unit and normally don't work well over large ranges oftemperature. This low cost radio altimeter does not needIso-Circulators.

Coupled to each of the low noise amplifiers is a bias regulator 2030.Low noise amplifier bias regulator 2030 provides a stable voltage sourcefor each of the three GaAs FET amplifiers used in the receiver of thelow cost radio altimeter of the present invention.

A monolithic microwave integrated circuit (MMIC) 2040 mixes the localoscillator signal with the amplified received signal and produces anoutput whose frequency is the difference between these signals. For thelow cost radio altimeter, a 40 HZ signal at the output of this MMICrepresents a height of 1 foot above the ground. Likewise an aircraftusing this device at 2000 feet above the ground would expect to see afrequency of 80,000 HZ at the output of the balanced mixer.

A high pass filter 2050 using resistors and capacitors is provided tohelp eliminate the effects of any radio altitude signals caused byleakage of the microwave signals. In one embodiment of the presentinvention, the altimeter uses double shielded coaxial cables; each threefeet in length; to feed the antennas. Any real altitude signal will havehad to travel out the 3 foot transmit cable, been reflected by theground, and then return through the 3 foot long receiver cable. Themicrowave signal travels approximately 1.5 times slower through thecables then it does through free space. Because of this, any radioaltitude signal with a frequency less than approximately 360 HZ willhave been caused by leakage, rather then by reflection of the signalfrom the ground. High pass filter 2050 significantly attenuatesfrequencies below 400 HZ, while offering minimum attenuation tofrequencies above this point.

Three low noise operational amplifiers 2060 are used to amplify the verylow level radio altitude signal coming from balanced mixer 2040.Amplifiers 2060 are contained within the metal shielding which housesthe microwave circuits and supply an output to an operational amplifier2070.

Operational amplifier 2070 receives the signal coming from the microwaveassembly and acts as a buffer for the incoming signal. Amplifier 2070also is used as an output driver for a low pass filter 2080.

Low pass filter 2080 is implemented with resistors and capacitors andprovides significant attenuation to frequencies above 160 KHZ. Somefrequency components at and above 160 KHZ could be actual radio altitudesignals indicating altitudes at or above 4000 feet. As the actualaltitude of the aircraft increases, the signal strength of the reflectedsignal decreases, and it becomes much more likely that signals at orabove this frequency will be the result of noise, rather than realsignal. Attenuating signals at and above this frequency helps insurethat the altimeter will correctly indicate an out of track conditionwhile flying at higher altitudes above the ground.

The output of low pass filter 2080 is supplied to operational amplifier2090. Operational amplifier 2090 receives the signal coming from lowpass filter 2080 and acts as a buffer. Additionally, amplifier 2090 isused as an output driver for comparator circuits 2110. Comparatorcircuit 2110 uses an analog comparator to convert the analog radioaltitude signal into a level suitable for digital logic circuits.

Frequency Modulated Continuous Wave (FMCW) radio altimeters usetriangular modulation wave forms to help eliminate Doppler shift and itsassociated inaccuracies. While this method solves one problem, itcreates yet another. At each point where the frequency sweep directionchanges, an ambiguity exists. The difference signal will decrease tozero frequency as the transmitter frequency changes direction. The timeduration of this accuracy corruption is fairly short and is a functionof how high above the terrain the aircraft is flying. A frequencyreversal window generator circuit 2100 uses digital logic circuits andsignals from the transmitter's modulation generation circuits togenerate a pulse or window starting at the instant when the modulationfrequency changes direction. This signal is approximately 1.024 milliseconds in duration and drives the base of a NPN transistor. This NPNtransistor is used to short the radio altitude signal to ground for1.024 milli seconds after the modulation signal changes direction.During this window when the altitude signal will be inaccurate, thealtimeter is prevented from processing the received signals.

A signal level detection comparator circuit 2120 converts the radioaltitude signal transitions into an equivalent DC voltage, to provide anindication that the signal strength is adequate to provide correct radioaltitude information to the pilot.

FIG. 9 contains a block diagram of the transmitter portion of the radioaltimeter.

Reference 2900 is a 1 MHZ crystal oscillator and digital frequencygeneration circuit which includes 1 MHZ crystal oscillator, binarycounters, and digital logic circuits; constructed to generate precisetiming signals needed for the low cost radio altimeter. This digitallogic circuit forms a state machine which produces the followingsignals:

(a) Precisely symmetrical square wave at the modulation frequency(either 100 Hz or 102 HZ as selected by a signal line available on theRF module's interface connector);

(b) A two times the modulation frequency short duration pulse used bythe receiver circuits to eliminate the ambiguity caused when themodulation direction is reversed. (see Frequency reversal windowgenerator in the receiver description); and

(c) 1 MHZ clock provided to the receiver circuits for the Frequencyreversal window generator.

A Level shifting amplifier and analog integrator 2910 receives as itsinput, the precisely symmetrical square wave from reference 2900 above.Since this square wave is generated from digital logic circuits, itsamplitude is subject to the tolerance normally allowed for in digitallogic circuits. This variation in amplitude from unit to unit couldultimately cause transmitters to be either under or over deviated infrequency. A lack of precision in modulating the transmitter results insignificant inaccuracies in the altimeter's altitude readout. Thisinvention minimizes modulation errors by generating a preciselysymmetrical square wave from a stable crystal oscillator reference, andby precisely controlling the amplitude of the modulation signal asdescribed below. The precisely symmetrical square wave is AC coupled (toeliminate any DC offsets from the digital circuits) to an operationalamplifier which has its gain set such that the output of the amplifierwill be driven to saturation on both the positive and negative swings ofthe input signal. Since the operational amplifier is powered fromregulated plus and minus supplies, the resulting saturated square waveis also referenced to the regulated supplies.

Upon leaving level shifting amplifier 2910, this saturated square wavenow has precision symmetry and frequency and additionally is now tightlycontrolled in amplitude. This signal is now applied to an analogintegrator which has an integration period much longer than the appliedsignal. The output of this integrator is a triangular wave form whichwill be used to modulate the microwave transmitter. Since theintegration period is much longer than the applied signal, normalvariations in the value of the integration capacitor will not affect thelinearity of the output. Variations in the value of capacitors areusually associated with changes in ambient temperature and long termaging of the component.

Microwave deviation and frequency adjust circuits, with Modulation poweramplifier 2920 receives triangular wave form described in reference 2910above. Triangular wave form output from 2910, is applied to a variableresistor which allows the amplitude of the triangular wave form to beadjusted on each individual RF module. This simple adjustment is used toset the frequency deviation for the transmitter. The adjustableamplitude triangular wave form is then applied to an operationalamplifier which has a variable resistor used to set the DC offset of theamplifier. The DC offset is used to set the center frequency of themicrowave oscillator. A bipolar NPN transistor is used in conjunctionwith an operational amplifier to provide adequate power to drive themicrowave oscillator.

As noted in reference 2920, amplitude of the modulating signal sets themicrowave transmitter's frequency deviation, and the DC level of themodulating signal sets the microwave transmitter's center frequency. Itis essential that these voltages and amplitudes remain unchanged overtime and temperature, after initial calibration. For this reason, aseparate voltage regulator 2930 is used to insure stable voltages forthe generation of the modulation signal.

The triangular modulating power output is then supplied to a microwaveoscillator 2940. Microwave oscillator 2940 uses a single GaAs FET.Oscillator 2940 produces approximately +10 dBM (10 milliwatts) of powerat 4.25 GHZ. to 4.35 GHZ. The oscillator is voltage controlled whichallows it to be frequency modulated.

The oscillator design uses micro strip technology and includes resistiveloading of micro strip elements to lower the "Q" of the circuits. Thisapproach allows the use of epoxy glass laminate as the substrate for thecircuits. Lowering the "Q" of the micro strip circuits increases thebandwidth of the circuits therefore making them much more tolerant oflaminate variations and FET parameter variations. This novel approachlimits the available gain or power which could be realized from thecircuits and devices. The minor disadvantage is that more stages will berequired to implement a radio altimeter system. Advantages of thisdesign approach include at least the following:

(a) Fabricated on inexpensive epoxy glass laminate;

(b) Very tolerant of normal material variations;

(c) Elimination of tedious fine tuning;

(d) Allows for the simple replacement of a failed microwave device; and

(e) Consistant performance unit to unit. Ease of Manufacturing.

A microwave buffer amplifier 2950 uses a single GaAs FET. Amplifier 2950provides isolation for microwave oscillator 2940 and additional gain forthe transmitter system. Additionally, amplifier 2940 makes use of amicro strip band pass filter in its output circuit to help ensure thatthe final transmitted signal will be compliant with FCC rules andregulations. The microwave buffer amplifier design uses micro striptechnology and includes resistive loading of micro strip elements tolower the "Q" of the circuits. This amplifier is over driven bymicrowave oscillator 2940. This excessive drive allows the circuit to beextremely tolerant of laminate variations and device variations, asdescribed in reference 2940 above.

The operating point for microwave buffer amplifier 2950 is set by thevoltage output of a microwave buffer amplifier bias regulator 2960. Thisis an adjustable output type regulator which provides a stable, isolatedand regulated voltage for buffer amplifier 2950.

Microwave transmitter power amplifier 2970 also uses a single GaAs FET.Amplifier 2970 makes use of a micro strip band pass filter in its outputcircuit to help ensure that the final transmitted signal will becompliant with FCC rules and regulations. The microwave transmitterpower amplifier design uses micro strip technology and includesresistive loading of micro strip elements to lower the "Q" of thecircuits. This amplifier is also over driven by the microwave bufferamplifier 2950. This excessive drive allows the circuit to be extremelytolerant of laminate variations and device variations, as described inreference 2940 above. The microwave transmitter power amplifier's outputis typically +18 dBm into 50 ohms at frequencies between 4.25 and 4.35GHZ. A small amount of the microwave transmitter's energy is coupledfrom the output via a micro strip coupler to be used as a localoscillator signal in the receiver.

The operating point for microwave transmitter power amplifier 2970 isset by the voltage output of the microwave transmitter power amplifierbias regulator 2980. This is an adjustable output type regulator whichprovides a stable, isolated and regulated voltage for microwavetransmitter power amplifier 2970.

A microwave signal detector circuit 2990 comprised of a micro stripcoupler from the microwave transmitter power amplifier's output and amicrowave detector diode. If the transmitter is producing sufficientpower, energy coupled by the micro strip coupler will be rectified bythe microwave detector diode. The resultant low level DC voltage is usedas an indication that the transmitter is functioning.

FIGS. 10A through 10F are illustrative of a schematic of microstriptechnology utilized in an embodiment of the present invention. Inductors3010, 3012, 3014, 3016, 3018, 3020, 3022, 3024, 3026, 3028, 3030, 3032,3034, 3036, 3038, 3040, 3042, and 3044 are microstrip inductors.Circuits 3110, 3112, 3114, 3116, 3118, 3120, 3122, 3124, 3126, 3128,3130, and 3132 are microstrip circuits. Printed wiring board material is0.032 inch FR4 (epoxy glass laminate). All resistors and physicalcapacitors are surface mount devices.

SOFTWARE CONTROL AND DEVICE OPERATION

Radio altitude data from the radio altimeter as converted to digitaldata are input to microprocessor 736 under the control of a RADIOALTITUDE INPUT routine 3312, illustrated in FIG. 11. Initially, two datasamples are obtained in steps 3314 and 3316. These radio altitudesamples are obtained when the microprocessor 736 asserts signals readthe high and low bytes of radio altitude data.

In step 3318 the system checks of the samples are valid. The validity ofthe radio altimeter signals are checked by checking the radio altimeterstatus signals that signify the availability of a positive and negativepower supply of the required voltage to the radio altimeter system. Athird signal is a tracking signal and, as mentioned above, is used toindicate when the radio altimeter subsystem is processing signalsindicative of radio altitudes greater than 3,000 feet.

In step 3322 the high and low bytes of the radio altitude data arecombined into a single value. During periods when the modulationfrequency changes, the counter is disabled for about 1.024 milliseconds.In order to compensate for this condition, the radio altitude datasignal is multiplied by about 1.257. Subsequently, the radio altitudedata and validity samples are stored in step 3322; and the systemreturns in step 3324 to repeat the process.

Should it be determined in step 3318 that the two radio altitude samplesgathered in steps 3314 and 3316 are invalid, an additional two samplesare gathered in step 3326. The new samples are checked for validity instep 3328. If these samples are valid, they are processed in step 3320as discussed above. If not, fail flags are set in step 3330 and thesystem returns to step 3320.

The radio altimeter data is processed by a radio altitude processingroutine 3332 (FIG. 12). As will be discussed in more detail below, theradio altitude processing routine 3332 is used primarily to determinethe closure rate based upon radio altitude data. As will be discussed inmore detail below, the closure rate is utilized for the "CAUTION RISINGTERRAIN" and "TERRAIN TERRAIN" call-outs.

In FIG. 12, the raw radio altitude and validity data discussed above isretrieved in step 3334. The raw radio altitude data is lowpass filteredin step 3336 and saved in step 3338 along with the validity dataobtained in step 3334. The closure rate in feet per minute is computedin steps 3342 and 3344. The closure rate is provided in feet per minuteand relates to the rate of change of radio altitude in feet as afunction of time. The closure rate is filtered in step 3344 and saved instep 3346. The system returns in step 3348.

The algorithm for the terrain warning system process in 3350 isillustrated in FIG. 13. The TWS processing system 3350 is a main programthat makes calls to the radio altitude input processing routine 3312, aswell as the radio altitude processing routine 3332. The TWS processingroutine 3350 is made up of a collection of independent software objects,illustrated in FIG. 14. A current value table 3370 keeps track of allinputs from the various objects.

As shown in FIG. 14, the TWS processing system includes a timer 3352that is used by a real-time task manager 3354 and a back-ground task(BG) manager 3356 for managing the tasks of all of the objects in thesystem. On power-up, the system does a number of start-up testsidentified in FIG. 13 as BITE functions 3360. The start-up tests includechecking the power supply 3362, checking the RAM 3364, the ROM 3366, aswell as resetting the watch dog timer 3368. Should any of the power-uptests fail, an INOP flag is written to a current value table 3362.

In FIG. 14, the system 3350 also receives discrete inputs handled by adiscrete input processing object 3376 from the radio altimeter subsystemas discussed above, as well as the flap and landing gear position. Inaddition, the system monitors the status of the system test switch 970,the audio inhibit switch 900, as well as the decision height up and downswitches 910. The inputs to the system are separated into twocategories: discrete inputs 3372; and radio altimeter subsystem havebeen discussed previously in connection with the radio altitude inputprocessing routine 3312.

The discrete inputting processing 3376 is responsible for processing theflap and gear signals as well as the switch signals discussed above. Inparticular, the flap and gear input signals are under the control of achip select signal driven by microprocessor 736. The inputs for thedecision height switches 910, the audio inhibit switch 900, as well asthe system test switch 970 are under the control of a control signalZCSSWITCH also driven by microprocessor 736. The decision height switch910 as well as the audio inhibit switch 900 and system test switch 970are applied to connection 996 which, in turn, is applied to anotheroctal latch. The output of this octal latch is applied to the data busunder the control of the switch chip select signal ZCSSWITCH. Thissignal allows microprocessor 736 to read the status of the switch inputsdiscussed above under the control of the real time task manager 3354. Inaddition to reading the status of the discrete and switch input signals,the discrete inputs object 3372, as part of the discrete inputprocessing routine 3376 debounces all inputs and stores the values forall inputs in the current value table 3370.

In FIG. 13, the TWS processing 3350 also includes display driverprocessing routine 3378. The display driver processing routine 3378includes various objects as illustrated in FIG. 14, including a decisionheight altitude object 3380, a discrete output arbiter 3382, a displaydriver 3384 and a discrete output driver 3386. In one embodiment of theinvention display 737 is under the control of the control signalsCSDISP1, CSDISP2 and CSDISP3. These signals are, in turn, driven byports or the microprocessor 736. In particular, the control signalCISDISP2 controls the 1000 s and hundreds digits of display 737, whilethe control signal CSDISP3 controls the 10s and 1s digits of the display737. Control signals CSDISP2 and CSDISP3 are used to drive theseven-segment decoders. The control signal CSDISP1 is used to control alatch used for various functions, including a lamp test.

During conditions when the radio altimeter system is out of track, the Gsegments of each of the characters are illuminated and the remainder isblanked such that all dashes are displayed to indicate to the pilot ofthe aircraft that the radio altimeter is out of track. In addition,anytime the system test switch 970 is depressed, and the radio altitudeis at or below 30 feet, a lamp test and all the LED segments areilluminated which is initiated under the control of the control signalCSDISP1 as discussed above. If the radio altitude is above 30 feet, testswitch 970 causes the TWS system to perform an internal test but doesnot change the displays.

The TWS system may be used to record certain data, such as radioaltitude data, decision height set altitude, time ticks used to indicatethe interval of time used to time tag the recorded events, the statustest, audio inhibit status, flap input, gear input, status of thealert/warning call-out generation status, as well as the status of theself-test switch. Such data is adapted to be stored in the flash memorydevice 745. The recording of this data under the control of the flighthistory routine 3388 includes the history object 3390, which, asmentioned stores data in flash memory 745 under the control of a flashobject 3392. Further details on the structure and operation of theflight history recorder are provided elsewhere in this specification.

As discussed in more detail below, the TWS system is adapted to providevarious Mode 2 and Mode 4 warnings by the routines 3394 and 3396. Thesewarnings are under the control of the Mode 2 and Mode 4 objects 3398 and3400.

In addition to the Mode 2 and Mode 4 warnings, the system is adapted toprovide Mode 6 altitude call-outs under the control of the routine 3402.The Mode 6 altitude call-outs are under the control of the altitudecall-out object 3404, as well as the voice arbiter 3406 and voice driver3408.

The audio subsystem is under the control of a control signal CSAUDIO,available via a port driven by microprocessor 736. As discussed above,the various warnings and call-outs and messages are stored in flashmemory device 745. These stored digital voice recordings are sequencedout of flash memory device 745 under the control of the control signalCSAUDIO.

The TWS processing system 3350 also includes a Mode 6 decision heightroutine 3406, which compares current values of the radio altitude by wayof a decision height minimum object 3408 and drives a voice driver 3408to provide the call-outs at a pilot selected decision height.

Switch 910, previously described, enables the pilot of the aircraft toselect the height at which the "MINIMUMS MINIMUMS" audio message isprovided. Anytime either of the decision height switch 910 is depressed,it causes the display to momentarily blank and subsequently indicate thecurrent setting of the decision height altitude. In one method forpracticing the invention, the decision height altitude is slowly flashedon and off to distinguish it from radio altitude, for example at aflashing rate of twice per second with an on/off ratio of 70/30 percent.Within two seconds of placing the decision height switch 910, display945 returns to displaying the radio altitude. Each time power is appliedto the system, the decision height is automatically set to 000 feet,which, in turn, disables the decision call-out. Pressing and holding thedecision and height switch 910 for at least two seconds causes thesystem to enter the decision height setting mode as discussed above. Thedisplay 745 will flash the current decision height altitude and willslowly start increasing or decreasing the displayed decision heightaltitude, depending on which switch is pressed. The decision heightmight be incremented or decremented at 50 foot intervals. When thedesired decision height is reached, the pilot merely releases theswitch. Each time the aircraft passes through the decision heightaltitude, a "MINIMUMS MINIMUMS" call-out is generated under the controlof the voice arbiter 3406 and voice driver 3408. The TTWMON processingroutine 3412 includes the objects TTYMON, CUTMIRROR, TRACELOG, SERIALBUFFER ARBITER, and the RS232 UART from FIG. 14. The TTYMON routineprovides a test interface along with data download/upload capabilitiesto the CVT. During laboratory testing and debug TTYMON is used to verifythat all switch objects inside the TWS are functioning correctly. Inaddition to lab testing TTYMON is used during flight testing to acquiresystem information to analyze TWS performance and improve the switch ifit is needed. TTYMON also provides the interface to download flighthistory stored in flash ROM by object 3390.

The radio altitude input processing, as well as the radio altitudeprocessing 3312 and 3332 have been discussed above. These routines areunder the control of the objects 3371 and 3374.

As mentioned above, the system provides various audio alerts and, inparticular, a "CAUTION RISING TERRAIN". The "TERRAIN TERRAIN" audiowarning is generated when the system detects terrain rising at ratesgenerally illustrated in FIG. 15. In a preferred embodiment of theinvention, the TWS system does not receive barometric rate data and istherefore unable to determine whether the closure rate computed is theresult of rising terrain, loss of barometric altitude or a combinationof both. As such, the warning envelope illustrated in FIG. 15 isselected to provide the aircraft with as much warning time as possible,while minimizing nuisance alerts. The audio messages are generated foreach penetration into the envelope shown in FIG. 15. The rising terrainaudio messages are generated at ten second intervals while the aircraftremains within the envelope. Exiting the alert envelope to the left orabove will silence the alert. If the situation worsens and the aircraftexits the warning envelope either to the right or down, the audiomessage is replaced with a "TERRAIN TERRAIN" warning. Immediately uponthe generation of the caution "RISING TERRAIN" audio message, the pilotis expected to verify correct aircraft altitude, attitude and geographiclocation. Selecting full flaps will eliminate the audio message.

The warning envelope generally indicates closure rates of 1000 feet perminute or greater at altitudes of 500 feet or less. The warning envelopeis anticipated to provide about 25 seconds of warning time to the pilot.In addition to the "TERRAIN TERRAIN" audio message, the altitudecall-outs will also be generated as the aircraft passes through thosealtitudes. The altitude call-outs have priority over the terrainwarning. These call-outs are provided once for each descent through thespecific altitude, if their associated enabling altitudes have beenreached. Arbitration between the various audio messages is accomplishedby the voice arbiter 3406. If the aircraft moves from the "CAUTIONRISING TERRAIN" envelope to the "TERRAIN TERRAIN" envelope, the "CAUTIONRISING TERRAIN" audio message will be terminated while the "TERRAINTERRAIN" audio messages will start.

The system also provides a "TOO LOW FLAPS" call-out. The aircraftdescends, for example, below 170 feet of radio altitude without fullflaps selected. This call-out is generated for each descent and will notbe enabled again until the aircraft has climbed above 500 feet of radioaltitude.

The system also provides a "TOO LOW GEAR" call-out for aircraft equippedwith retractable gear. If the aircraft descends, for example, below 450feet of radio altitude without the gear extended, this voice message isenunciated to the pilot. The message issues only if enabled, e.g., afterthe aircraft has climbed above 800 feet of radio altitude.

AIRCRAFT FLIGHT HISTORY FUNCTION

As briefly described above, the flight safety device of the presentinvention may be used to record certain flight data thereby preserving arecord of the flight. This type of data and the safety benefits itprovides has heretofore been unavailable to the pilot/operator ofgeneral aviation aircraft. Specifically, this data proves useful inaccident reconstruction and investigation, and the monitoring of studentpilots during solo flights. Furthermore, the flight history data mayalso yield improvements in the alert functions of the present inventionby providing information on the performance of the warning envelopes andthe occurrence of nuisance warnings. In the absence of this recordedinformation, improvements can only come through the subjective andhappenstance medium of direct pilot feedback.

FIG. 16 contains a flow chart describing operation of the flight historyfunction according to an embodiment of the present invention. Thewriting of flight history data to flash ROM 745 is controlled throughactivation of a switch 3600. Activation of switch 3600 is governed bythe generation of an alert or warning, detection of a change in aircraftstatus, and/or at predetermined time intervals.

In step 3602 of FIG. 16, the device reads the status of the externalinputs. These inputs may include, but are not limited to: flaps, gear,altitude signal, audio inhibit, and decision height selected. The statusinputs of step 3602 are used by the flight safety system to generatewarnings, alerts and altitude call-outs in step 3605 according to theprocedures described in the previous section. According to a preferredembodiment of the invention, the flight history device includes aprovision to enable switch 3600 whenever a microprocessor 736 outputs awarning or alert. The switch enable is depicted in step 3610 of FIG. 16.Activation of switch 3600 by step 3610 causes the warning or alert datadevice status to safety device status to be written to flash ROM 745.Flight safety device status data may include: self test initialized bythe crew, audio output detected, radio altitude status bits, powersupply input power status and power supply supply output voltages. Thesystem status data may be present within the system as a discretevariable, a status bit, the output of a latch or according to any numberof techniques commonly known to those of skill in the art.

Operation of switch 3600 may also be triggered by detecting a change inthe aircraft or system status. Process 3615 represents the status inputsdescribed in the previous paragraph. In step 3620, the outputs ofprocesses 3602, 3605 and 3615 are summed. A change in sum indicates achange in either the aircraft or system status 3625, enabling switch3600 and causing the system status data and flight data to be written tomemory 745.

In the case of radio altitude data, a change in status is determined bydetecting a change of predetermined magnitude from the previous value.The predetermined magnitude can be a variable range depending upon theheight above ground level. The variable range results in fewer updatesto the flight history when the aircraft is flying at higher altitudesand more frequent updates when the aircraft is flying at loweraltitudes. Additionally, the variable range permits the update rate tovary according to the type of aircraft on which the device is installed.

In the absence of change of status or warning data to trigger switch3600, various other conditions as represented by block 3630 may beestablished to enable operation of switch 3600. These conditions may,for example include:

(a) flight at a predetermined altitude above ground;

(b) type of aircraft;

(c) aircraft mission;

(d) time since last update; and/or

(e) warnings recently issued. Other criteria for enabling switch 3600may be used.

In the preferred embodiment of the invention, the flight history memory745 comprises a 2 Meg boot block type flash ROM, Model No.TE28F200BX-T80 manufactured by Intel. Alternatively, a 4 Meg version ofthis memory device may be used. Sector erase flash memories can also beused but tend to have higher densities and associated costs. Pasttechnologies of EPROM and EEPROM have the desired nonvolitility butposses certain undesirable characteristics for use in the presentinvention. EPROMS cannot be reprogrammed within the system. EEPROMS lackthe desired density and are too slow to be used in the present device.Furthermore, both EPROMS and EEPROMS do not posses the same facility forrewriting and erasing as does the boot block type flash ROM.

The boot block flash ROM is subdivided into blocks of variable length bythe manufacturer. The variable block size proves advantageous over themore regularly subdivided sector erase or bulk erase type memories.Specifically, the boot block flash ROM include the feature of a lockable"boot" sector for storing program code. The boot sector requiresexecution of additional steps to reprogram and is not easilyoverwritten. The lockable boot sector can be used by the presentinvention to enable processor 736 to be run out of this block while thesystem code/voice memory ROM is reprogrammed in the field via the RS232connection.

The remaining blocks of ROM 745 are employed by the present invention asfollows. As data is written to the ROM, the end of one of these blocksis reached. The microprocessor is able to skip to the next availableblock and begin writing at that location. As clear blocks are used,microprocessor 736 initiates an erase of the oldest memory block. Theerase does not effect the remaining blocks.

In general, the last five minutes of the flight are retained in memory,however any interval may also be used including continuous recordation.In addition, recording of data can be stopped when the radio altitude isbelow 30 feet or above 3,000 feet. Each of the status entries are timetagged. The time ticks are reset at power-up and are sequential untilpower-down.

Data from several flights fit into a single block. This storage capacitypossesses unique advantages over prior art flight recorders. Inparticular, the extended flight history would prove of value to flightinstructors evaluating student solo performance. The extended flighthistory also provides additional information to accident investigators.Alternatively, the data from several flights may be used to show trendinformation on a particular pilot or aircraft.

According to one additional aspect of the present invention, device 2may be automatically configured during installation. "Auto configurable"flap, gear and audio volume select inputs read in any flap or gear inputvoltages and store the flap and gear up and down positions in the flighthistory FLASH 745.

In "manual" flap and gear input methods a separate input for eachvariation of input expected exists. For example, one aircraft may havean input with flaps up=28V and flaps down=0V. Another aircraft may haveflaps up=0V and flaps down=12V, and yet another could have flaps upgrounded and flaps down open. Each of these type of inputs requiresseparate input pins or some type of configurable input using "programpins". Audio volume select has also been done with program pins in thepast.

The manual configuration has been a problem in past designs because asnew aircraft types were added to the list of installable aircraft,software and hardware changes were usually required. There arepotentially an infinite number of variations of input ranges given thenumber, types and age of the target aircraft for TWS. In theautoconfigure mode, the TWS has only one flap input and one gear inputand no program pins.

The elimination of program pins is accomplished using an A/D converter,software, Flash EPROM, and the device front panel displays and switches.The A/D converter reads in whatever voltage is present at the flap orgear inputs. The device installer will be prompted to place theflaps/gear up and down and the TWS device will store the up and downposition readings in one of the Flight History 745 flash segments. Ifthe aircraft has no flaps or gear that information will be stored aswell. The Audio volume will be set for a particular aircraft in much thesame way; the installer will be prompted to select an appropriate volumewhile the TWS device outputs a test message.

The front panel displays and switches will be used for implementing amenu system for installation configuration and test. For example, thedecision height up and down 910 can be used to adjust the audio volumeup and down, the altitude display can display the current input voltage,and the decision height display can be used for operator prompts.

Preferred embodiments of the invention have been described. Variationsand modifications will be readily apparent to those of skill in the art.For this reason the invention is to be interpreted in light of theclaims.

What is claimed is:
 1. A radio altimeter comprising:a predeterminedoscillator for generating an oscillation signal at a predeterminedfrequency; means for modulating said oscillation signal with apredetermined modulation signal defining a modulated signal; means forradiating said modulated signal defining a radiated modulated signal;first means for coupling said modulated signal to said radiating means,said first coupling means including one or more radiating circuitmicrostrips; means for amplifying said modulation signal, saidamplifying means having a predetermined gain selected to eliminate theneed to tune said one or more radiating circuit microstrips; means forreceiving a reflection of said radiated modulated signal definingreflection signals; means for mixing said modulated signals with saidreflection signals to generate an audio signal whose frequency isproportional to the altitude of the aircraft above ground; second meansfor coupling said reflection signals to said mixing means, said secondcoupling means including one or more receiving circuit microstrips; andmeans for amplifying said reflection signals, said amplifying meansincluding a plurality of low-noise amplifiers, at least one of saidlow-noise amplifiers having a predetermined gain selected to eliminatethe need to tune said one or more receiving circuit microstrips.
 2. Aradio altimeter as recited in claim 1, wherein said predeterminedoscillator is a voltage controlled oscillator.
 3. A radio altimeter asrecited in claim 2, wherein said voltage-controlled oscillator includesa GaAs field-effect transistor (FET).
 4. A radio altimeter as recited inclaim 1, wherein said predetermined modulation signal is a triangularwave having a predetermined modulation frequency.
 5. A radio altimeteras recited in claim 4, further including means for enabling manualselection of said modulation frequency from a plurality of modulationfrequencies.
 6. A radio altimeter as recited in claim 1, wherein saidmodulation signal amplifying means includes a predetermined bufferamplifier.
 7. A radio altimeter as recited in claim 6, wherein saidmodulation signal amplifying means includes a power amplifier coupled tosaid buffer amplifier by way of said coupling means.
 8. A radioaltimeter as recited in claim 6, further including means for adjustingthe operating set point of said buffer amplifier.
 9. A radio altimeteras recited in claim 7, further including means for adjusting the setpoint of said power amplifier.
 10. A radio altimeter as recited in claim1, further including means for amplifying said reflection signals.
 11. Aradio altimeter as recited in claim 10, wherein said reflection signalamplifying means includes a low-noise amplifier (LNA) coupled to saidmixing means and coupling means for coupling said LNA to said mixingmeans, said coupling means including one or more microstrips.
 12. Aradio altimeter as recited in claim 1, further including means forproviding a signal representative of the gain of said modulationsignals.
 13. A radio altimeter comprising:an oscillator for generatingan oscillation signal at a predetermined frequency; a modulator forgenerating a modulation signal for modulating said oscillation signal,said modulation signal being a triangular wave at a predeterminedmodulation frequency, defining a modulated signal; a buffer amplifierhaving a predetermined gain for amplifying said modulated signal, saidbuffer amplifier being electrically coupled to said oscillator, saidcoupling including one or more microstrips; a power amplifier having apredetermined gain, electrically coupled to said power amplifier, saidcoupling including one or more microstrips; an RF output terminalelectrically coupled to said power amplifier, said coupling includingone or more microstrips; an RF input terminal for receiving reflectedsignals; a mixer for mixing said modulated signals with said reflectedsignals to generate a signal whose frequency is proportional to thealtitude of the aircraft above ground; a local oscillator couplingdevice which includes one or more microstrips for coupling saidmodulated signal to said mixer; and a plurality of low-noise amplifiers,each of said amplifiers having a predetermined gain, said amplifierselectrically coupled to said RF input terminal and to said mixer bymicrostrips, wherein the gain of at least one of said amplifiers isselected to eliminate the need to tune said microstrips.
 14. A radioaltimeter as recited in claim 13, further including means for adjustingthe operating point of said buffer amplifier.
 15. A radio altimeter asrecited in claim 13, further including means for adjusting the operatingpoint of said power amplifier.
 16. A radio altimeter as recited in claim13, further including means for adjusting the operating point of atleast one of said low-noise amplifiers.
 17. A radio altimeter as recitedin claim 13, wherein said circuit for generating said modulation circuitincludes a counter for generating a square wave having a predeterminedmodulation frequency and means for integrating said square wave togenerate a triangular wave modulation signal.
 18. A radio altimeter asrecited in claim 17, further including means for disabling saidmodulation signal for predetermined frequencies.
 19. A radio altimeteras recited in claim 13, further including means for detecting the outputlevel of said modulated signal and comparing it with a reference signalto generate a status signal representative of the level of the outputsignal.
 20. A radio altimeter as recited in claim 13, wherein said radioaltimeter is formed on a glass/epoxy circuit board.
 21. A frequencymodulated continuous wave radio altimeter comprising:(a) a transmitcircuit for transmitting a first signal; (b) a receive circuit forreceiving a reflection of said first signal and for producing a secondsignal indicative of a height above ground; and (c) at least one of saidtransmit circuit or said receive circuit including:(i) at least onemicrostrip circuit; and (ii) a plurality of amplifiers electricallycoupled to said microstrip circuit, said amplifiers having a gainselected to eliminate the need to tune said microstrip circuit.
 22. Thealtimeter of claim 21 wherein said altimeter is fabricated on aglass/epoxy printed circuit board.
 23. The altimeter of claim 21 whereinsaid amplifiers further include GaAs field effect transistors as activeelements.